Sources of subsurface contamination are numerous, for example, leaky underground storage tanks, industrial and manufacturing operations, chemical storage and process areas, chemical spills and waste disposal areas. Among common contaminants from these sources are petroleum hydrocarbons, such as benzene, toluene and xylene, gasoline, diesel, jet fuel and others; chlorinated hydrocarbons, such as trichloroethylene (TCE), tetrachloroethylene (PCE), chlorobenzene and chlorophenols; and other volatile, semi-volatile and non-volatile organic compounds. Once such contaminants are within the vadose zone they can leach down into the groundwater table and become long term sources of groundwater contamination which typically persist for decades. The vadose zone, also known as the zone of aeration, is the unsaturated zone of the soil that is above the groundwater table and the capillary fringe (i.e., the transition zone between the vadose zone and the groundwater table). Typically, the capillary fringe has about a 75% to about a 90% water content and the vadose zone has from about a 30% to about a 50% water content. In order to protect groundwater resources and provide unrestricted use of land and groundwater, clean-up of subsurface contamination is necessary at many sites.
One well known process in the field of subterranean environmental clean-up is the process of vacuum extraction. [See U.S. Pat. No. 4,593,760 by Visser and Malot, issued Jun. 10, 1986, on which a Reexamination Certificate was granted Jun. 20, 1989; and U.S. Pat. No. Reissue 33,102 by Visser and Malot, issued Oct. 31, 1989.] The vacuum extraction process removes volatile and semi-volatile organic compounds, petroleum hydrocarbons, and other liquid contaminants from the vadose zone. In the vacuum extraction process, a subsurface vacuum enhances volatilization of volatile organic compounds in the subsurface, and the vacuum induced air flow removes these volatilized contaminants from contaminated soil.
The vacuum extraction process typically works faster on compounds of higher vapor pressure (i.e., greater than or equal to 1 mm Hg at 20.degree. C.) than those of lower vapor pressure (i.e., less than about 1 mm Hg at 20.degree. C.). Compounds with low vapor pressures have been removed successfully by vacuum extraction, but at a slower removal rate than that of higher vapor pressure volatile organic compounds. In cases where the subsurface is contaminated with a dense non-aqueous phase liquid (DNAPL), such a compound, being more dense than water, tends to sink deep into the subsurface and even into or below the groundwater aquifer. Once the DNAPLs are in or below the groundwater aquifer, they are generally considered untreatable.
Because of its oxidation potential, hydrogen peroxide, and more specifically, the hydroxyl radical, is known to be an effective treatment method for removal of contaminants from soils and waste streams. [See Schneider, D. R. and Billingsley, R. J., "Bioremediation--A Desk Manual for the Environmental Professional," Pudvan Pub. Co., Northbrook, Ill., pp. 60-61 (1990); Watts, R. J., Solomon, W. L., and Udell, M. D., "Treatment of Contaminated Soils Using Catalyzed Hydrogen Peroxide," Dept. of Civil Eng., Washington State Univ. (1990); Watts, R. J., Udell, M. D., Rauch, P. A. and Leung, S. W., "Treatment of Pentachlorophenol-Contaminated Soils Using Fenton's Reagent," Hazardous Waste & Hazardous Materials, Vol. 7, No. 4 (1990); and Watts, R. J., "Hydrogen Peroxide for Physicochemically Degrading Petroleum-Contaminated Soils," Remediation Magazine, pp. 413-425 (1992).] The oxidation potential of hydrogen peroxide is also a well known phenomenon and has been studied since the turn of the century. The basic reaction is the oxidation of an organic molecule, such as a hydrocarbon, phenol or a chlorinated compound, to form a variety of oxidized products. [See Sedlak, D. L. and Andren, A. W., "Oxidation of Chlorobenzene with Fenton's Reagent," Environ Sci Technol , Vol 25, No. 4 (1991); and Walling, C. and Johnson, R. A., "Fenton's Reagent. V. Hydroxylation and Side-Chain Cleavage of Aromatics", Dept. of Chemistry, Univ. of Utah (1974).] In a complete oxidation reaction, one by-product might be carbon dioxide (CO.sub.2), or if the oxidation is an incomplete reaction, then by-products might include alcohols, aldehydes or carboxylic acids, all of which are very biodegradable. In the present invention the oxidation of subsurface contaminants by hydrogen peroxide generates no toxic byproducts, is environmentally benign, and the hydrogen peroxide itself may be degraded by subsurface microbial enzymes.
Hydrogen peroxide reacts with iron (Fe.sup.2+) to produce hydroxyl radical (OH.multidot.) which is commonly referred to as Fenton's reaction (other possible catalysts are copper and nickel). Fenton's reaction produces the hydroxyl radical which, being a strong oxidizer, can be utilized to treat subsurface contaminants. Fenton's reaction is commonly written as: EQU H.sub.2 O.sub.2 +Fe.sup.2+ .fwdarw.OH.multidot.+OH--+Fe.sup.3+.
This type of reaction is catalyzed when hydrogen peroxide contacts naturally occurring iron contained in soil and rock. Remediation of contaminated soils and waste streams has been done on a bench scale by means of Fenton's reaction. (Supra, Watts et al. (1992).] Watts et al. applied Fenton's reagent (pre-mixed iron and hydrogen peroxide) to excavated, contaminated soils contained in open drums. The results showed that soil contamination levels of several thousand milligrams per kilogram (ppm) of total petroleum hydrocarbon were reduced to below 100 mg/kg within a few days. However, the prior art does not address methods to capture the offgases which are produced from the oxidation reaction.
Most prior art methods of treating contaminated soil involve excavation and treatment of the soil on- or off-site by means such as incineration or chemical treatment. However, when soil contaminated with volatile organic compounds is excavated, up to about 90% of the contaminants volatilize to the atmosphere. Many of these prior art methods are ex situ and most all of these methods release the volatilized contaminants into the atmosphere with the disadvantages attendant thereto, such as adverse environmental or health impacts or other effects. Not only are these prior art methods expensive, but also, in many cases the prior art methods are not practicable. For example, if the contaminated soil is beneath developed or arable land, then excavation of the soil for treatment is not a viable alternative. However, one widely recognized technique for in situ clean-up of soil contaminated with volatile organic compounds is the vacuum extraction process of Visser and Malot, supra. These two patents issued to Visser and Malot do not recognize nor suggest the synergy between the oxidation step and the vacuum extraction step of the process of the present invention which results in a more efficient clean-up of contaminated media. In the process of the present invention, contaminated media may include, but are not limited to, soil, groundwater, waste streams, landfills, rock, etc.
Every contaminated media site has its own unique characteristics with regard to the type of contaminant, the complexities of the subsurface topography, the soil and rock location, etc. Accordingly, it is impossible to generalize or predict from laboratory experiments, for example, those of Watts et al., supra, what will happen in a clean-up situation at a particular contaminated media site.
In contrast to most of the prior art methods, the present invention is an in situ process for removal of subsurface contaminants. In the process of the present invention, the contaminants are subjected to oxidation. The heat produced by the oxidation reaction causes volatilization of the contaminants. These contaminants are then withdrawn from the subsurface by means of vacuum extraction. The process of the present invention is advantageous, because the process is performed in situ and, therefore, is more cost effective than prior art methods which require excavation of the contaminated soil. Additionally, the process of the present invention is environmentally benign, because the volatilized contaminants can be, preferably, recovered and treated and not released into the atmosphere.